A variety of approaches to actuating MEMS devices have been described. Some approaches use magnetic fields to pivot a moving member relative to a substrate. One such approach is described in U.S. Pat. No. 5,912,608 to Asada, entitled PLANAR TYPE ELECTROMAGNETIC ACTUATOR and U.S. Pat. No. 5,767,666 to Asada et al., entitled PLANAR TYPE ELECTROMAGNETIC ACTUATOR INCORPORATING A DISPLACEMENT DETECTION FUNCTION, each of which is incorporated herein by reference. Other approaches use electrostatic forces to pivot a moving member or to drive a sliding piece relative to a substrate. Examples of such devices can be found in U.S. Pat. No. 5,629,790 to Neukermans et al., entitled MICROMACHINED TORSIONAL SCANNER and U.S. Pat. No. 5,867,297 to Kiang et al., entitled APPARATUS AND METHOD FOR OPTICAL SCANNING WITH AN OSCILLATORY MICROELECTROMECHANICAL SYSTEM, each of which is incorporated herein by reference.
Among the applications for such MEMS devices are scanning beam imaging, including image acquisition and display. In image acquisition, such MEMS devices typically include a mirror that pivots to sweep a beam through a prescribed scanning field. A detector in the imaging device collects reflected light and produces an electrical signal in response. A processor then identifies image information from the electrical signal. Equipment incorporating such devices can be found in barcode readers, image capture systems, confocal imagers, and other applications.
Scanning beam displays, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference, are one approach to overcoming many limitations of conventional displays. As shown diagrammatically in FIG. 1, in one embodiment of a scanned beam display 40, a scanning source 42 outputs a scanned beam of light that is coupled to a viewer's eye 44 by a beam combiner 46. In some scanned displays, the scanning source 42 includes a scanner, such as scanning mirror or acousto-optic scanner, that scans a modulated light beam onto a viewer's retina. In other embodiments, the scanning source may include one or more light emitters that are rotated through an angular sweep.
The scanned light enters the eye 44 through the viewer's pupil 48 and is imaged onto the retina 59 by the cornea. In response to the scanned light the viewer perceives an image. In another embodiment, the scanned source 42 scans the modulated light beam onto a screen that the viewer observes. One example of such a scanner suitable for either type of display is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference.
Sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user's field of view and presents an image that occupies a region 43 of the user's field of view 45, as shown in FIG. 2A. The user can thus see both a displayed virtual image 47 and background information 49. If the background light is occluded, the viewer perceives only the virtual image 47, as shown in FIG. 2B.
As shown diagrammatically in FIG. 3, the scanning source 42 includes an optical source 50 that emits a beam 52 of modulated light. In this embodiment, the optical source 50 is an optical fiber that is driven by one or more light emitters, such as laser diodes (not shown). A lens 53 gathers and focuses the beam 52 so that the beam 52 strikes a turning mirror 54 and is directed toward a horizontal scanner 56. The horizontal scanner 56 scans the beam 52 periodically in a sinusoidal fashion. The horizontally scanned beam then travels to a vertical scanner 58 that scans periodically to sweep the horizontally scanned beam vertically. For each angle of the beam 52 from the scanners 56 and 58, an exit pupil expander 62 converts the beam 52 into a set of beams 63. Eye coupling optics 60 collect the beams 63 and form a set of exit pupils 65. The exit pupils 65 together act as an expanded exit pupil for viewing by a viewer's eye 44. One such expander is described in U.S. Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporated herein by reference.
Returning to the description of scanning, as the beam scans through each successive location in the beam expander 62, the beam color and intensity is modulated in a fashion to be described below to form a respective pixel of an image. By properly controlling the color and intensity of the beam for each pixel location, the display 40 can produce the desired image.
Simplified versions of respective electrical waveforms for vertical and horizontal scanning are shown in FIGS. 4A and 4B. Responsive to the electrical waveforms, the beam traces the pattern 68 shown in FIG. 5 in an image plane, such as the plane of beam expander 62 of FIG. 3. Though FIG. 5 shows only eleven lines of image, one skilled in the art will recognize that the number of lines in an actual display will typically be much larger than eleven.
As can be seen by comparing the actual scan pattern 68 to a desired raster scan pattern 70, the actual scanned beam 68 can be “pinched” at the outer edges of the beam expander 62. That is, in successive forward and reverse sweeps of the beam, the pixels near the edge of the scan pattern are unevenly spaced. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because the image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern 70, the displaced pixel locations can cause image distortion. Some approaches to treating such distortions or image imperfections are described in U.S. Pat. No. 6,140,979 to Gerhard, et al. entitled SCANNED DISPLAY WITH PINCH TIMING AND DISTORTION CORRECTION, which is incorporated herein by reference.
For a given refresh rate and a given wavelength, the number of pixels per line is determined in the structure of FIG. 3 by the mirror scan angle θ and mirror dimension D perpendicular to the axis of rotation. For high resolution, it is therefor desirable to have a large scan angle θ and a large mirror. However, larger mirrors and scan angles typically correspond to lower resonant frequencies. A lower resonant frequency provides fewer lines of display for a given period. Consequently, a large mirror and larger scan angle may produce unacceptable refresh rates for many MEMS scanners.